The present invention relates to printed circuit boards for forming a diplexer circuit, more particularly but not exclusively to a diplexer circuit for a base station transceiver of a mobile communication system.
Modern base station transceivers need to be operated at high efficiency levels to provide users of a mobile communication system with high data rate and quality services while keeping the power consumption and radiated power at a possible minimum. Therefore, power amplifiers (PA for abbreviation) and low noise amplifiers (LNA for abbreviation), building the high-frequency (HF for abbreviation)-front-end of a base station, should experience as low noise and interference as possible. One possible source of noise and interference is a potential connection or crosstalk of the transmitter branch with the PA and the receiver branch with the LNA through a commonly used antenna, i.e. parts of the transmit signal may occur in the reception branch. Therefore, respective filter circuits can be utilized to attenuate such crosstalk.
One example of such a filter circuit is a so-called diplexer, which is a passive device that implements frequency domain multiplexing. Communication systems may us time division duplex (TDD for abbreviation) or frequency division duplex (FDD for abbreviation). In TDD, transmission and reception are separated in the time domain, i.e. for a given time either transmission or reception is carried out at the base station transceiver. In FDD, transmission and reception are separated in the frequency domain, i.e. different frequency bands are used for transmission and reception. Diplexers can be based on two band-pass filters, separating the transmission band from the reception band in an FDD system. Two ports (e.g., L and H, as abbreviations for low and high frequency band) are multiplexed onto a third port (e.g., S as abbreviation for signal), which connects an antenna or antenna system. The signals on ports L and H occupy disjoint frequency bands. Theoretically, the signals on L and H can coexist on port S without interfering with each other.
Typically, the signal on port L may occupy a single low frequency band and the signal on port H may occupy a higher frequency band. In that situation, the diplexer can consist of a low-pass filter connecting ports L and S and a high-pass filter connecting ports H and S.
Ideally, all the signal power on port L is transferred to the S port and vice versa, and all the signal power on port H is transferred to port S and vice versa. Ideally, the separation of the signals is complete, i.e. none of the low band signal is transferred from the S port to the H port. In the real world, some power will be lost, and some signal power will leak to the wrong port.
The isolation between the transmit-frequency band and the receive-frequency band in a FDD radio system is a very essential performance requirement for a base station transceiver. Due to the very high sensitivity of the receiver and the relatively much larger output power, the isolation, which is usually required between these two bands may be in the order of 70, 80, 90 or even 100 dB.